The present invention relates to a blood pressure waveform correcting device for producing an accurate blood pressure waveform in a blood monitoring system.
Blood pressure monitoring systems are employed in the medical field for monitoring, at all times, the blood pressure of a patient which varies from time to time. Such blood pressure monitoring systems as well as systems for controlling anesthesia are relied upon in surgical operations, for example.
In a blood pressure monitoring system, a catheter placed in a blood vessel of a patient is connected to a pressure transducer through a blood pressure measuring line including a pressure transmitting tube and a pressure dome, and the pressure transducer is coupled to a CRT display unit or a recorder unit.
The catheter placed at a point for measuring the blood pressure of the patient and the blood pressure measuring line are filled with a physiological saline solution for the following reasons:
If air were left trapped in the catheter or the blood pressure measuring line, since air which is highly compressible is of large compliance, it would obstruct proper blood pressure transmission and also vary frequency characteristics of the blood pressure. To avoid this problem, priming is performed to remove air from the catheter and the blood pressure measuring line before the catheter is inserted into a blood vessel.
To effect priming, a physiological saline solution is injected via one port of a three-way cock in the pressure transmitting tube until the physiological saline solution overflows the catheter and an outlet port of the pressure dome. The physiological saline solution thus filled in the catheter and the blood pressure measuring line is used as a pressure transmitting medium. The blood pressure which is transmitted via the physiological saline solution is applied to a diaphragm in the pressure dome, and the applied pressure is converted to an electric signal by the pressure transducer. The blood pressure signal can directly be monitored or displayed on the CRT display unit or recorded on the recorder unit.
To allow for easy handling in a measuring process, the pressure transmitting tube comprises a flexible tube such as a soft tube of vinyl chloride or the like. Since the tube wall of the pressure transmitting tube pulsates in resonance with the pulsation of the patient's heart, the physiological saline solution in the pressure transmitting tube is subjected to an additional pressure developed by the throbbing tube wall as well as the blood pressure of the patient. This extraneous throbbing pressure is also transmitted to the diaphragm in the pressure dome, and will be observed as a distorted blood pressure waveform on the CRT display unit.
More specifically, since the pressure transmitting tube filled up with the physiological saline solution is present between the catheter and the pressure transducer, the blood pressure monitoring system has its own damping factor D and resonance frequency f due to the physical properties of the catheter, the pressure transmitting tube, and the pressure transducer. The blood pressure monitoring system is normally underdamped with the damping factor D ranging from 0.2 to 0.3. Because the damping factor D and the resonance frequency f are not of appropriate values, the blood pressure waveform as measured by the pressure transducer is distorted as compared with a desired blood pressure waveform. The resonance of the system is of a frequency which is several times higher than the frequency of the blood pressure, and has small vibration energy.
The damping factor D is defined as: ##EQU1## where X.sub.1, X.sub.2 are the amplitudes shown in FIG. 1 of the accompanying drawings. The resonance frequency f is expressed by f=1/t where t is the time (sec.) of one cycle shown in FIG. 1.
The damping factor D can be obtained by applying a pressure of 300 mmHg, for example, to the blood pressure measuring line, abruptly opening the three-way plug in the blood pressure measuring line, measuring the amplitudes X.sub.1, X.sub.2 of a waveform displayed on the CRT display unit, and putting the measured values X.sub.1, X.sub.2 in the above equation (1). If the damping factor D and the resonance frequency f are not appropriately related to each other, no accurate blood pressure waveform can be measured. A range in which the damping factor D and the resonance frequency f are of suitable relationship and an accurate blood pressure waveform can be measured varies with the pulse rate of the patient. As shown in FIG. 2, such a range for a patient having a pulse rate of 60 is on the righthand side of the curve E, and such a range for a patient having a pulse rate of 120 is on the righthand side of the curve F, the curves E, F being produced clinically. The best range for the damping factor D is from 0.4 to 0.6, including areas E.sub.T, F.sub.T on the curves E,
Various means for removing the effect of the resonance on the blood pressure measurement have been proposed
(1) In order to prevent a fluid from moving due to resonant vibrations, a variable resistance means is inserted in series in the blood pressure measuring line
(2) A membrane having a suitable degree of compliance is incorporated in the blood pressure measuring line to increase a damping factor due to the resiliency of the membrane.
(3) A variable resistance means and an air chamber are placed in the blood pressure measuring line.
With the proposal (1) above, however, the variable resistance means can be inserted only in a limited position in the blood pressure measuring line. More specifically, since the variable resistance means is placed in series in the blood pressure measuring line, if the variable resistance means were inserted in a wrong position, then the resistance means would make it impossible to flush the blood pressure measuring line. More specifically, the blood pressure measuring line is filled with a pressure transmitting line such as physiological saline solution. The physiological saline solution is forced to flow in a small amount into the patient's blood vessel in order to prevent the blood from flowing into and being solidified in the blood pressure line. In addition, a device is disposed in the blood pressure measuring line for flushing the line to remove foreign matter and blood from the line. Where the variable resistance means is inserted in the blood pressure measuring line according to the proposal (1), if a solution container for supplying the physiological saline solution, the variable resistance means, and the flushing device are arranged in this order, a desired amount of the flushing solution cannot flow downstream in the line because the resistance means is disposed upstream of the flushing device. When setting the variable resistance means in place, the resistance of the variable resistance means is progressively increased while looking at the CRT display unit to observe the manner in which the effect of the resonance is eliminated. Thus, delicate adjustments are required to determine a position to set the variable resistance means,
The arrangement (2) above is effective in increasing the damping factor D due to the resiliency of the membrane, but reduces the resonance frequency f of the blood pressure measuring line, so that it will be difficult to measure accurate blood pressure waveforms.
According to the system (3) above, delicate adjustments are also required to establish a flow passage through the variable resistance means to the air chamber when setting the variable resistance means and the air chamber in place, and such delicate adjustments are time-consuming.